1. Field of the Invention
The present invention relates to a magnetooptic record reproducing device, and more particularly, to a magnetooptic data record reproducing device which can reproduce magnetooptically recorded data.
2. Discussion of the Related Art
In current systems for recording information with a magnetooptic disk device, a laser beam spot is focused on a surface of the disk, to elevate a temperature of the a film of magnetic recording medium (for example, an alloy of an amorphous rare metal (R) and transition such as TeFeCo or Gb--Tb--Fe) to a temperature of 150-200 deg. C. The laser beam being of a size commensurate with a wavelength modulated into a coded data to be recorded. The recording medium is demagnetized when the temperature of the recording medium is elevated to a temperature above a Curie temperature Tc by the heat from the laser beam. A direct current bias magnetic is applied to the recording medium in one direction using a magnet to cause a magnetization inversion at the heat portion. When the heated portion off the recording medium cools to room temperature, a magnetic domain is recorded. On the other hand, the coded data thus recorded on the disk can be reproduced by focusing a direct current laser beam of predetermined power to a spot size in a range of a wavelength. As a polarity of the laser beam reflected at the surface of the magnetooptic recording medium is changed according to the Kerr effect, the polarity of the reflected laser beam is optically detected to read information magnetized on the disk. In the aforementioned art, which has been put into practical use already in 3.5" or 5.5" magnetooptical disk drive, a spot diameter D of the focused laser beam can be expressed as D-0.5w/Na, where w is a wavelength of the laser beam and NA is a numerical aperture of a lens. Therefore, reading a magnetic pit with a spot of a diameter D-0.62.mu. is hardly possible optically, if the wavelength of a semiconductor laser beam w-0.68.mu. and the numerical aperture of a lens NA 0.55.
JP Laid Open Patent No. H3-93058 discloses a super resolution reproducing method as a method for reading a magnetic pit d(&gt;D) smaller than an optical spot diameter D, in which a laser beam scans a disk rotated by a power higher than a general reproducing power and information of the magnetic pit smaller than the spot of the laser beam is read utilizing a difference of a temperature distribution of a magnetic film between a central portion and an outer portion of the laser beam spot. And, JP Laid Open Patent No. H5-20697 discloses a method of optical super resolution, in which a center portion of a focusing lens is shielded with an aperture for making the spot diameter D(-0.5w/NA), which is decided according to a wavelength w and numerical aperture NA, smaller. However, this method can not be put into practical use due to reason such as significant affect from side lobes formed in periphery of the focused spot in recording and reproducing and lack of laser power in recording due to the shielding of the center portion of the lens with an aperture.
The problems in the conventional art will be explained with reference to FIGS. 4a and 4b, taking a magnetooptic disk recording system with a laser pulse direction and magnetic field modulation as an example.
FIG. 4a illustrates a sample servo system. A laser element 1 is made to emit a pulsed laser beam (having a waveform represented by reference number 2) under control of a laser driving device 9 in response to a clock signal 10 generated from phase pits in the disk. An objective lens 3 directs an optical spot 4 onto a magnetooptic recording medium 8. In the meantime, a data signal generating device 6 generates a modulation magnetic field 7 using a magnetic head 5 disposed close to the disk. If pulses of the optical spot 4 of the laser beam are directed onto a surface of the disk with a higher frequency of the clock signal 10, the laser beam 2 pulsating synchronized as to the clock 10 and the modulation magnetic field 7 cause the successive optical spots 4 to overlap to cause over write recording, thereby recording with magnetic pits 11 (FIG. 4B) of mark lengths smaller than the diameter D of the optical spot 4.
This method is made known by JP Laid Open Patent No. 111-292603. For example, even though the diameter D=0.62.mu. of when the wavelength w" 0.68.mu. and the lens numerical aperture NA=0.55, the reduction of pulse intervals makes recording of the shortest mark length D=0.1.about.0.2 .mu. possible. Currently, when a track pitch p=0.6.mu. and the shortest mark length d-0.26, a capacity of information recordable on a disk of a diameter 120 mm is 7.about.10 GB (giga byte). However, reproduction of the magnetic data thus recorded with the shortest mark length d-0.1.about.0.2.mu. using the optical spot with a diameter D=0.62.mu.(-0.5w/NA) is very difficult.
FIG. 3a illustrates a magnetic super resolution method for reading a magnetic pit of a size smaller than an optical spot 4, made known by JP Laid Open patent No. H3-93056. A magnetooptic film 12 has 2-4 layers of magnetooptic films having magnetic and thermal properties different from one another (recording layer 12-1, reproducing layer 12-2 and switching layer 12-3). When the optical spot 4 scans a track 16 having a magnetic domain recorded within the magnetooptic film 12 as shown in FIG. 3b, photo energy is absorbed by the magnetic medium and converted into heat to form a temperature distribution within the optical spot 4. As a result, in the high temperature region of the spot 4, the switching layer 12-3 is heated to a temperature in the vicinity of a Curie temperature (T.sub.c .congruent.140 deg. C.), a switched connection force between the recording layer 12-1 and the reproducing layer 12-2 is weakened, and a direction of magnetization of the reproducing layer 12-2 which has a lower coercive force becomes in agreement with the reproducing magnetic field 14. As a result, within the optical spot 4, only a magnetic pit 15 in a low temperature region 13-2 of the spot 4 is read because a magnetic pit 15-1 in the high temperature region 13-1 is masked by the reproducing layer 12-2. However, in this method, although a signal level from a 0.4.mu. mark length can achieve a target level of 45 dB, if the mark length is less than 0.3.mu., the signal level becomes 30 dB only.